A digital radiographic detector may include an array of pixels. An exemplary unit passive pixel schematic diagram 300 is shown in FIG. 3. It includes one PIN photodiode 302 and one thin film transistor (TFT) 304. The PIN photodiode 302 stores charge proportional to the light exposure it receives as provided by an adjacent x-ray scintillator. The TFT 304 is used as a switch that transfers the charge from the PIN photodiode 302 onto a common column data line 306 which eventually is electrically connected to the detector's readout circuitry. The gate 308 for the TFT receives a control signal from an electrically connected common row select signal source where one or more rows can be accessed simultaneously.
A TFT in a passive pixel may use amorphous silicon (a-Si) technology. These TFTs are known to be stable with respect to optical illumination, environment, radiation and electrical stress. Oxide TFTs, such as Indium Gallium Zinc Oxide (IGZO), may show instability with respect to optical illumination, environment, radiation and electrical stress. One performance metric common to some oxide TFTs is a threshold voltage (VT) shift over time which influences the effective TFT turn on voltage. In the case of x-ray radiation instability, it has been shown that the VT will shift to the left (decrease) with increasing x-ray exposure over the life of the pixel.
A VT compensation circuit may be an effective way to compensate for VT shift due to environmental effects, such as temperature, and TFT process variability. An x-ray detector may include a TFT array over a large area panel which requires good uniformity and consistent performance. Oxide TFTs may show left VT shift dependence over increasing x-ray exposure, and this cumulative x-ray exposure may not be uniform across the detector's entire array of pixels over its lifetime. Having an in-pixel VT compensation circuit would be an effective way to reduce such a left VT shift.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.